Even though we cannot see sound, it is a force with real dimensions and definite properties: intensity and duration. Intensity is the loudness of a sound, or the pressure it exerts through the ear. It is measured in units called decibels (dB). In assessing noise, a special measure called "dBA" indicates damage to hearing. Examples of the dBA rating for many common exposures are provided in the first table. The higher the dBA number, the greater the risk of damage to hearing.
|140||threshold of pain: gunshot, siren at 100 feet|
|135||jet take off, amplified music|
|120||chain saw, jack hammer, snowmobile|
|100||subway (not the sandwich shop)|
|90||OSHA limit - hearing damage if excessive exposure to noise levels above 90 dB|
|85||inside acoustically insulated tractor cab|
|75||average radio, vacuum cleaner|
|45||rustling leaves, soft music|
|30||whisper, empty theater, watch ticking|
|15||average threshold of hearing|
|0||acute threshold of hearing - weakest sound|
Duration is the amount of time you are exposed to a sound level. In the second table, the right-hand column lists various high sound levels and the left-hand column indicates the length of exposure that is safe for the corresponding noise level during a day. These figures have been determined after years of research on noise-induced hearing loss and are accepted as the standard for allowable noise level exposures.
|Duration - hours per day||Sound level (dBA)|
|1/4 or less||115|
The ears provide two warning signs for over-exposure to noise: temporary threshold shift (TTS) and ringing in the ears (tinnitus). After leaving a noisy area or piece of equipment, many people commonly experience both of these symptoms. The temporary hearing loss is difficult to detect unless a hearing test is performed.
This temporary hearing loss was taken into consideration in the exposure limits listed in the second table. For example, should you be exposed to a noise level of 100 dBA for two hours, the remaining 22 hours of that day's exposure should be at a noise level below 90 dBA. This allows the ear to recover from the temporary hearing loss. This recovery period varies, depending upon the individual, the severity and length of exposure.
Hearing usually returns almost completely in 12 to 14 hours if there is no more noise exposure. Any amount of hearing that does not return becomes a permanent threshold shift (PTS) or permanent hearing loss. With repeated exposure, the effects are cumulative. It is the need to prevent this cumulative damage that drives hearing protection programs, including the one here at UCSC. If you suspect that you are being exposed to excessive levels of noise, contact EH&S. We can perform 8-hour time-weighted monitoring as well as spot measurement of noise levels to determine what level of protection is appropriate to protect your hearing.
List of links about noise and hearing conservation